-------- Original Message --------
Subject:        re rectangular maps.
Date:   Fri, 7 Mar 2008 05:35:01 +0000
From:   Brendan Whyte <[log in to unmask]>
To:     Maps, Air Photo & Geospatial Systems Forum <[log in to unmask]>




Mary,

http://www.ncgia.ucsb.edu/education/curricula/giscc/units/u014/tables/table02.html
is a table of the length (in km and miles) of a degree of longitude at
various latitudes from the equator (0 lat) to the poles (90 lat)
The mainland US basically fits between 30 and 50 degrees latitude north,
so from southern Texas, where a degree of longitude is 60 miles, to the
49th parallel, where it is about 45 miles, the length of a degree of
longitude shrinks by 25%.
In Alaska, which is basically between 60 and 70 degrees Lat north, a
degree of longitude shrinks from 35 miles to 24 miles.

A degree of latitude is basically 69 miles long (it does vary *slightly*
due to the earth not being a perfect sphere).
http://www.ncgia.ucsb.edu/education/curricula/giscc/units/u014/tables/table01.html

So a map of 1 degree long by 1 degree lat  will show 60 miles E-W and 69
miles N-S for southern Texas, but 45 miles E-W and 69 N-S for the
Canadian border.
! In Alaska is will cover 35 miles E-W and 69 miles N-S near Anchorage
and only 24 miles E-W and 69 miles N-S near Barrow

For a 7'30" x 7'30" sheet, divide these distances by 8 (7'30" is
one-eighth of a degree (60 minutes in a degree))

To prove these shrinking distances, show the patron lines of longitude
on a globe and how they converge at the poles.


Dr Brendan Whyte
Ubon Ratchathani University
THAILAND


 > > Hello folks,
 > > I think that some of you out there will be able to come up with a well
 > > thought out answer for this question before I can think of where to
look!
 > > "Why on a 7.5 minute USGS Topo isn't the area map square? Since the
map is
 > > 7.5 minutes X 7.5 minutes, it should be square. The map clearly is not
 > > square and appears rectangular. Can you help with this dilemma?"
 > > I've told the patron that I'm sending the question out to the
"experts".

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